1) an assembly of machines which, in order to achieve the same end, are arranged and controlled so that they function as an integral whole, notwithstanding that the machines may themselves be relevant machinery and accordingly severally required to comply with the relevant Regulations; or2) interchangeable equipment modifying the function of a machine, which is supplied for the purpose of being assembled with a machine or with a series of different machines or with a tractor by the operator himself save for any such equipment which is a spare part or tool.